Just Blizzard being lazy I guess? Or just forgot to update it, either way, it is the old Archimonde's Darkness talent, which was named something else with Archimonde so it just needs to be updated

Edit: The Talent calculator is also very buggy, the other talents are marked for Demonology no matter what I do, and things like Mannoroth's Fury does not show a duration either. Soul Leech is also at it's pre-nerfed state and so on.